Conventionally, for power device related applications (60-2000 V), silicon based power MOSFETs or IGBTs are used. In addition to high-voltage and high-current capability, these devices should also have low on-state power losses and good switching characteristics (e.g. fast switching with minimal switching losses etc.). However, at present neither of these Si devices offers an ideal combination of the aforementioned specifications. Specifically, a Si MOSFET has very good switching characteristics but for high-voltage applications, its on-resistance becomes very high. This limits usage of Si MOSFETs only for applications that require devices with a breakdown voltage (V.sub.B) of less than 600-900 V. On the other hand, even for devices with high V.sub.B (600-2000 V), Si IGBTs have very good on-state characteristics (low forward voltage drop at high current-density). However, Si IGBTs can be used only for low-frequency applications (&lt;40 KHz) because at high switching frequencies the switching losses for IGBTs become too high for practical applications. Thus, in the present day Si technology there is no single device that can offer combined benefits of Si MOSFET (fast switching, MOS gate control etc.) and Si IGBT (low forward voltage drop for high V.sub.B applications).
Recently, for power device applications silicon carbide (SiC) has gained a lot of attention due to its large electric field strength, high thermal conductivity and reasonably high mobility. It is expected that SiC based MOSFETs and IGBTs would be able to offer significantly improved performance advantages over their Si counterparts. For example, unlike Si technology where MOSFETs cannot be used for application that require devices with V.sub.B greater than 900 V, SiC MOSFET are expected to be useful for up to 2500 V applications. However, for higher V.sub.B applications (2500-5000 V), the on-resistance of a SiC MOSFET becomes too high and SiC based bipolar devices start offering performance advantages. Notably, compared to Si IGBTs and thyristors, SiC IGBTs could be operated at much higher switching frequencies due to their lower carrier lifetime as well as smaller drift regions.
Over the last five years, many different power transistors based on SiC technology have been demonstrated. These devices include SiC based MOSFETs, IGBTs and thyristors. Some of these devices have exhibited highly encouraging results in terms of the low on-state losses, high switching speeds, and high operating temperature capability. However, none of these devices come anywhere close to realizing the full potential that an optimally designed SiC based power device is expected to offer. One reason for the below expected performance of the experimental devices demonstrated so far is that all of these devices are essentially minor variations on conventionally used Si power devices. The design of these earlier devices does not address performance issues (e.g. poor inversion layer mobility and poor high-temperature reliability of gate oxide) that are specific to SiC technology.
For example, double diffusion MOSFET (DMOSFETs) is one of the most commonly used power MOSFET structure in Si technology but in its present form it cannot be used in SiC technology. In a DMOSFET gate-control occurs through an inversion channel that is formed in p-conductivity material along the lateral surface. However, due to the lack of manufacturable diffusion technology for silicon carbide, DMOSFETs cannot be fabricated in SiC. Also, the inversion channel in the DMOSFET is provided by forming gate oxide on p-conductivity material between the source and gate and for SiC this results in a poor quality of oxide with high fixed charges and a large number of traps at the oxide/SiC interface. This results in a reduction of the mobility of the carriers (electrons) that produce the current in the device and this reduction in the mobility of the electrons severely degrades the ON-resistance of the device.
An alternative, vertical structure for silicon carbide is a UMOSFET disclosed in U.S. Pat. No. 5,233,215, entitled "Silicon Carbide Power MOSFET with Floating Field Ring and Floating Field Plate" and issued Aug. 3, 1993. In the UMOSFET an inversion channel is formed by an MOS gate along an etched trench. Similar to DMOSFET, in the UMOSFET, the gate oxide is again formed on a p-conductivity layer, which produces a poor quality oxide and high ON-resistance for SiC based FETs.
Another major problem with SiC MOSFETs or IGBTs based on DMOS or UMOS technology is that due to the large breakdown field strength of SiC, in these devices the electric field in the gate oxide is very high. Experimental studies suggest that due to the high-temperature oxide reliability concerns in SiC MOS devices, the electric field in the oxide should be contained below 4 MV/cm. However, this would require limiting the electric field in SiC drift region to be much below the inherent breakdown field strength of the material. This suggests that for the case of SiC IGBTs devices based on DMOS or UMOS technology, the device performance (breakdown voltage, on-resistance etc.) will be determined by the gate oxide reliability concerns and not due to the intrinsic properties of SiC.
Accordingly, it would be highly advantageous to have a manufacturable IGBT with low ON-resistance, good switching characteristics (e.g. fast switching times and minimal switching losses etc.), low leakage currents, high channel density, etc.
It is a purpose of the present invention to provide a new and improved IGBT.
It is another purpose of the present invention to provide a new and improved IGBT with low ON-resistance , good switching characteristics, low leakage current, and high channel density.
It is yet another purpose of the present invention to provide a new IGBT structure that minimizes the electric field in the gate oxide and thus, alleviates the concerns of the gate-oxide reliability at high-temperature and high electric field.
It is a further purpose of the present invention to provide a new and improved IGBT which can be fabricated in silicon or silicon carbide or any III-V material system.